Interstellar Ti ii in the Milky Way and Magellanic Clouds☆

We discuss several sets of Ti ii absorption-line data, which probe a variety of interstellar environments in our Galaxy and in the Magellanic Clouds. Comparisons of high-resolution [full width at half-maximum (FWHM) similar to 1.3-1.5 km s-1] Ti ii spectra of Galactic targets with corresponding high-resolution spectra of Na i, K i and Ca ii reveal both similarities and differences in the detailed structure of the absorption-line profiles - reflecting component-to-component differences in the ionization and depletion behaviour of those species. Moderate-resolution (FWHM similar to 3.4-4.5 km s-1) spectra of more heavily reddened Galactic stars provide more extensive information on the titanium depletion in colder, denser clouds - where more than 99.9 per cent of the Ti may be in the dust phase. Moderate-resolution (FWHM similar to 4.5-8.7 km s-1) spectra of stars in the Magellanic Clouds suggest that the titanium depletion is generally much less severe in the Large and Small Magellanic Clouds than in our Galaxy [for a given N(H(tot)), E(B - V), or molecular fraction f(H(2))] - providing additional evidence for differences in depletion patterns in those two lower-metallicity galaxies. We briefly discuss possible implications of these results for the interpretation of gas-phase abundances in quasi-stellar object absorption-line systems and of variations in the D/H ratio in the local Galactic interstellar medium.